This invention relates generally to non-destructive inspection of structures and, more particularly, to methods and apparatus for non-destructively inspecting for debonding and delamination faults in composite materials and for structural faults in monocoque and semi-monocoque structures.
Advances in modern composites have made it possible to introduce high strength/weight ratio materials into highly stressed critical components such as those used in aircraft and weapons, for example. The development of alternative resin systems has also greatly improved the flexibility of manufacturing such structures. Tough, high strength, high temperature thermoplastic polymers combined with advanced graphite fibers have made it possible to consider structures of a size and strength heretofore considered economically impractical. In the aircraft industry, for example, where composite materials are increasingly being used, the size of composite structures range from relatively small control surfaces on drones, targets and missiles to the very large surfaces of vertical stabilizer, aileron and wing control elements on military and commercial aircraft; future applications include fuselage structures.
Such composite materials have failure modes quite different from metals and alloys in that they suffer from delamination, debonding and cracking, as well as deterioration from fatigue and ambient exposure. In addition to failure in service, these materials may also suffer from defects in manufacturing similar to those which occur under field stresses. Thus, a practical technique is needed for non-destructively locating faults in composite structures which is suitable not only for in-plant non-destructive evaluation but for field use as well.
Among known techniques for detecting delamination or debonding is that known as acoustic "scrubbing" wherein a small instrument containing an acoustic crystal is manually scanned across the surface of the object under examination while "listening" for anomalies in the sound. For small objects, this process is simple and efficient, but is much too time-consuming, and therefore prohibitively expensive, to be practical for evaluation of large components and structures.
Another suggested technique is to heat part or all of the surface of the component by flash discharge lamps and after a predetermined delay making an infrared photograph of the surface with a CCD camera system. This system is premised on the assumption that decay of the surface temperature during the delay period is a function of the thickness and thermal conductivity of the material, factors which would be affected by debonds or delaminations and cause anomalies in the photograph. The effectiveness of this system is highly dependent on the surface finish or condition of the object.
Perhaps the simplest, yet most effective, technique that has been proposed is to simply tap the surface of the structure at different spots and listen to the resulting sound; a dead or hollow sound indicates a fault condition, as opposed to a sharp sound in a fault-free area. This technique, too, is very time-consuming and the evaluation is very subjective, making it impractical for evaluation of most structures.
Faults occurring in monocoque and/or semi-monocoque structures, basically a skeleton over which a skin is secured and of which an aircraft fuselage is a practical example, are manifested in much different ways than in composite structures. If the skin develops cracks, or is weakened or becomes detached from the skeletal framework, then a rupture can result causing impairment of the entire structure, particularly if it is an aircraft fuselage which is usually pressurized. Similarly, if the underlying skeletal structure cracks, or becomes corroded, or the fastening system fails, structural failure can result.
The difficult task of inspecting the substructure of the skin of an aircraft fuselage for corrosion, broken fastenings, cracks or loose rivets is effectively performed by the method described in applicant's copending U.S. patent application Ser. No. 08/011,911 filed Feb. 1, 1993. The therein disclosed non-destructive inspection method includes the steps of mechanically exciting, with an exciter affixed to the object, a vibrating nodal pattern in the outer skin of a monocoque structure, panels of which may freely vibrate at one or more resonant frequencies. The frequency of excitation is scanned through a spectrum which includes such resonant frequencies and is locked onto a resonant frequency which is uniquely characteristic of a certain phase relationship of the anti-nodes of the nodal pattern. The amplitude of the excitation is then varied until the anti-nodes of the pattern are optimized to be either bi-concave or concave/convex, and when such optimization occurs two time-displaced holograms of the vibrating outer skin are recorded in synchronism with maximum plus and minus displacement, respectively, of an anti-node. It is from this fringe map of contour patterns, which is uniquely characteristic to any given structural condition, that faults are interpreted.
The speed and accuracy of analysis of the results produced by this vibrational technique for detection of flaws is improved by the system described in U.S. patent application Ser. No. 08/108,123 filed by applicant and another on Aug. 17, 1993, the disclosure of which is incorporated herein by reference. It is a computer-based system which analyzes nodal patterns induced in the surface of a structure being inspected either by analyzing a record obtained by interferometry, or by directly scanning, or digitally interrogating point-by-point, the vibrating surface of the structure with a beam of quasi-coherent radiant energy and, utilizing the Doppler effect, reading the out-of-phase displacement of the surface to produce a visual contour map of the surface for capture and computer analysis. Alternatively, a computer may be programmed to either directly capture the nodal pattern and provide either a direct readout of the location and/or identity of faults, or directly capturing the nodal pattern for later processing.
Unfortunately, the just-described techniques for locating faults in semi-monocoque structures do not always effectively reveal debonding and delamination faults in composite materials, and, conversely, known techniques for non-destructively inspecting composite materials are inapplicable to semi-monocoque structures, in addition to being less than ideal for their intended usage. There is therefore a need for a non-destructive evaluation technique, suitable for both in-plant and field use, for non-destructively inspecting for faults in composite materials, semi-monocoque structures and monocoque structures, large or small.